Asfarviridae
Asfarviridae is a family of icosohedral enveloped viruses whose genome consists of a single molecule of linear double-stranded DNA of about 150000-190000 nucleotides long. The name of the family is derived from African Swine Fever And Releted Viruses. African Swine Fever Virus (ASFV) is the type species of the Asfivirus genus and is the sole member of the family. Recently, porcine CD163 polypeptide has been surmised by implication to be the cellular receptor for African swine fever virus (ASFV) (Sanchez-Tones et al., 2003)
Arteriviridae
Viruses of the family of Arteriviridae includes equine arteritis virus (EAV), lactate dehydrogenase-elevating virus (LDV) and simian hemorrhagic fever virus (SHFV). The Arterivirus having the greatest economic importance is Porcine Reproductive and Respiratory Syndrome Virus (PRRSV).
PRRSV
Porcine Reproductive and Respiratory Syndrome (PRRS) is one of the most economically important diseases of swine. The syndrome appeared almost simultaneously in North America and in Western Europe in the late 1980s, and has since spread to become endemic in the major swine producing nations of Europe, Asia, and the Americas. The etiologic agent of PRRS is a virus that has been designated PRRS virus or PRRSV. For both European and North American PRRS, the disease is characterized by reproductive failure in sows and gilts (late term abortions, still births, and mummies), high mortality among nursery pigs, and respiratory disease in swine of all ages. The disease has been the subject of recent reviews (Mengeling and Lager, 2000; Murtaugh et al., 2002; Nodelijk, 2002; Plagemann, 2003).
In the pig, PRRSV infection is limited to a subset of cells of the monocyte/macrophage lineage. Fully differentiated porcine alveolar macrophage (PAM) cells are the primary target cells for viral replication (Duan et al., 1997a; Duan et al., 1997b). Immortalization of PAM cells is technically challenging, and when successful, has resulted in cell lines that are not permissive for PRRS virus growth (Weingartl et al., 2002). PRRS virions are specifically bound by macrophages and internalized in clathrin-coated pits by endocytosis. Release from endocytic vesicles requires acidic pH (Nauwynck et al., 1999). Initial binding of virions is mediated by interaction of the viral matrix protein with heparin sulfate glycosaminoglycans (Delputte et al., 2002). Internalization can be facilitated by a 210 or 220 kDa membrane glycoprotein, as incubation of PAM cells with monoclonal antibodies to this polypeptide blocks infection with PRRS virus (Duan et al., 1998; Wissink et al., 2003). The 210 kDa glycoprotein has recently been identified as sialoadhesin, a member of siglec family of sialic acid binding immunoglobulin-like lectins (Pensaert et al., 2003). Transfection of the non-permissive PK-15 (porcine kidney) cell line with porcine sialoadhesin conferred the ability to internalize PRRSV particles, but there remained an apparent block at the uncoating stage, as virions entered into cellular vesicles but did not undergo nucleocapsid disintegration and vesicle membrane fusion. Viral genes were not expressed, and the transfected PK-15 cells were not rendered permissive for the PRRS virus (Vanderheijden et al., 2003). To our knowledge, transfection with sialoadhesin has not been shown to be sufficient to convert any PRRSV non-permissive cell line to a PRRSV-permissive phenotype.
Apart from primary porcine cells of the monocyte/macrophage lineage, the only other cell type known to be permissive for the growth of PRRSV in cell culture is the immortalized monkey kidney cell line MA-104 (Chladek et al., 1998) and derivatives such as MARC-145 (Kim et al., 1993) and CL-2621. It is not known why this one particular cell line is permissive, yet other mammalian cell lines are not. The PRRS virus binds specifically to a number of different cell types, but does not initiate infection (Kreutz, 1998; Therrien et al., 2000). In MARC-145 cells, the internalization of the virus by endocytosis and subsequent uncoating in low pH vesicles seems to mimic similar events in PAM cells (Kreutz and Ackermann, 1996). However, a number of monoclonal antibodies that bind to porcine sialoadhesin fail to detect a homologous protein on the surface of MARC-145 cells (Duan et al., 1998; Wissink et al., 2003), suggesting that MARC-145 cells may use a divergent member of the same protein family or a different receptor altogether.
Current PRRSV vaccines are propagated on simian cell lines, which is a potentially dangerous activity. The use of simian cell lines for vaccine production has the potential to introduce primate viruses of significance into swine lines intended for xenotransplant purposes. Because swine are being increasingly explored as a source of xenotransplanted organs for humans, the introduction of primate cell lines to swine populations may ultimately pose a risk to humans receiving xenotransplanted organs. Thus, it would be prudent to avoid the use of simian cell lines in swine vaccine preparations. It would be therefore desirable to identify or generate non-simian cells or cell lines capable of supporting PRRSV replication. Towards this goal, it is essential to identify the gene product(s) which may be responsible for conferring the permissivity for PRRSV replication as seen in certain simian cells lines as well as PAM cells. Once such gene products are identified, non-permissive cells might be rendered permissive by, for example, transfection of the essential gene into them, thereby affording a wider array of production lines for a vaccine.
One lab has reported that the tetraspanin protein CD151 from MARC-145 cells, when transfected into non-permissive BHK-21 cells, confers permissivity to the PRRS virus (Kapil and Shanmukhappa, 2003; Shanmukhappa and Kapil, 2001). This observation has yet to be confirmed by an independent lab.
We describe here an unrelated polypeptide, which when introduced into non-permissive cells, confers permissivity to the PRRS virus.